Evidence is mounting that Australia's native sea lion population has levelled off and may be declining, prompting a leading CSIRO scientist to call for more intensive research into the species.
The call, by Dr. Peter Shaughnessy, principal research scientist with the Division of Wildlife and Ecology, follows a joint study between CSIRO and National Parks and Wildlife South Australia.
Found only in Australia, the Australian sea lion has its principal breeding colonies at Kangaroo Island and Dangerous Reef, near Port Lincoln, in South Australia. Smaller colonies exist on the west coast of SA and in Western Australia.
"Data collected suggests to us that too many pups are dying and
we don't really know why," says Shaughnessy. "For example in
1996 at two colonies we found mortality rates of 30 per cent and 50 per
cent respectively."
The following year mortality decreased to only four per cent among Dangerous
Reef sea lions but the worry is that we have no idea what's causing these
dramatic fluctuations in pup mortality, says Dr Shaughnessy.
Possible causes of death include parasites, such as hookworm, environmental perturbation causing food shortage for lactating female sea lions, high rates of aggression by adult males, direct human harassment or by interacting with fishing gear.
Another possibility, says Dr Shaughnessy, is that sea lions may be facing stronger competition for food from their cousin, the New Zealand fur seal, whose numbers are increasing on Kangaroo Island at an annual rate of around 10%.
"The Australian sea lion is an endemic species which we need to
take care of. It's a tourist icon, up there with the koala, which attracts
tens of thousands of visitors to the sea lions' colonies every year. If
we don't find out why their numbers are not increasing, we may face more
serious problems later on," he says.
Dr Shaughnessy, recognised as an international authority on sea lions and
fur seals, suggests the Australian sea lion's population in 1998 may be
no greater than it was before European settlement of Australia.
One of the complicating factors in trying to solve the mystery of why sea lion numbers are not increasing is their unique breeding cycle. Of the 35 pinniped (seal) species in the world, the Australian sea lion is the only type which does not have a strict, annual breeding cycle.
A female sea lion will give birth to a pup after an 18 month cycle, meaning that her pupping season occurs in alternate summers and winters, or in alternate springs and autumns.
While calling for more scientific study of Australian sea lions to try to maximise their breeding potential, Dr Shaughnessy has welcomed the steady growth of NZ fur seal numbers, especially in South Australia.
He says it is clear from his team's most recent mark-recapture (count) of NZ fur seals in the summer of 1998 that the species is recovering from its overharvesting by sealers in the last century and early 20th century.
"Each year we return to South Australia we find greater numbers and new colonies, some of them admittedly quite small, and we should all be thankful that these wonderful creatures are making a healthy comeback," says Dr Shaughnessy.
NZ fur seals are also found in Western Australia, off southern Tasmania and occasionally in New South Wales.
Dr Shaughnessy has carried out his research in conjunction with National Parks and Wildlife South Australia staff based at Kangaroo Island and Port Lincoln.
More information from:
Dr Peter Shaughnessy Phone: (02) 6242 1760
Principal Research Scientist
peter.shaughnessy@dwe.csiro.au
David Salt Phone: (02) 6242 1645
Communication Coordinator Division of Wildlife and Ecology
david.salt@dwe.csiro.au